1. Field of the Invention
This invention relates to improvements in a zoom lens for ordinary photography having a zoom ratio of 2 or greater including a standard angle of view (about 46.degree.) and a zoom lens for finite distance having a zoom ratio of 2 or greater.
2. Description of the Prior Art
Describing first the lens for ordinary photography, it has been usual to use a zoom type comprising two concave and convex groups in designing a zoom lens capable of magnification change from a wide angle range to a telephoto range. This two-group type is suitable for providing a wide angle of view, if it attempts to cover the telephoto range, considerable difficulties are encountered in terms of abberrations and correction of spherical aberration becomes impossible and the fluctuation of distortion by zooming becomes great and thus, it has only been possible to realize a zoom ratio of 2 at best in order to obtain a compact zoom lens of high performance. Also, even if use is made of a positive-negative-positive three-group type capable of withstanding the need for a certain degree of wide angle of view and securing a relatively great zoom ratio, the focal length of the divergent group must become short in order to realize a compact lens system and this is unavoidably undesirable in terms of aberrations. Particularly, as regards distortion, essentially negative distortion occurs in the second group and in addition, along with zooming, the variation in position of light rays passing through the divergent group which is the second group becomes so great that there occurs a fluctuation of distortion, and it has been difficult to correct it.
On the other hand, aside from the field of photographing chiefly objects at infinity, there are fields such as micro photography in which photography is effected at a predetermined finit distance, macro photography, photograph enlargement, plate making, etc., and devices using zoom lenses in the cameras, readers, printers, etc. for these fields are also being announced. However, in these zoom lenses, the imaging performance required is very high and therefore, it is difficult to maintain a performance not inferior to that of fixed focus lenses over the entire magnification range and particularly, correction of distortion has been difficult. In zoom lenses for finit distance, imaging performance is important, but the disadvantage that the amount of movement of each group from the maximum magnification to the minimum magnification becomes too great must be overcome, and such disadvantage becomes greater as the magnification range approaches one-to-one magnification. In this respect again, two-group zoom lenses are not suitable. The above-mentioned three-group zoom lenses can obtain a great zome ratio by a relatively small amount of movement of each group, bit if it is attempted to increase the zoom ratio, the diameter of any of the groups becomes too great, and not only it has been difficult to secure a sufficient back focal length, but also it has been difficult to well correct distortion over a wide range of magnification variation.